1. Field of the invention
This invention relates to a process for the production of a length of continuous thin two-phase stainless steel strip having excellent superplasticity and surface properties from molten two-phase stainless steel by casting.
2. Description of Prior Art
It is already known from, for example, Trans. Quart. A.S.M. 61 (1968), 85 that some kinds of two-phase stainless steel have superplasticity. With a new process employing superplasticity, objects having a complicated shape can be manufactured with less machining time than with a conventional process because of low stress, and high flexibility in machining, resulting from the superplasticity. It is recognized that in order for two-phase stainless steel to exhibit superplasticity, it is necessary that it have a fine-grained texture.
In addition, it is reported in Nikki New Material No. 5, 1986, p. 30, that two-phase stainless steel develops a fine-grained texture having superplasticity when it is quenched and solidified into about 1 mm thick plate consisting of a ferrite phase only, cold-rolled to 80% of its thickness, and then annealed at 1050.degree. C.
As stated above, conventional production processes have to convert a molten two-phase stainless steel into plate, and then subject the plate to a heat treatment, so that a pure ferrite phase or a small amount of austenite remains in a ferrite matrix. This procedure has disadvantages because the heat treatment has to be conducted at elevated temperatures, a combination of repeated process is needed, and the production yield is low.
It is an object of the present invention to provide a direct production process of a length of continuous thin two-phase stainless steel strip having excellent superplasticity, and surface properties as cast.